The present invention relates to a new zeolite of the mordenite type and to a process for preparing this zeolite.
Because of their geometric selectivity and ion exchange properties, zeolites are utilized in industry on a large scale, in adsorption (for example gas drying, separation of linear and branched paraffins, separation of aromatic compounds, etc.) as well as in catalysis (for example catalytic cracking, hydrocracking, isomerization, oligomerization, etc.).
The chemical composition of the zeolites containing in their structure the elements Si and Al can be represented by the following approximate formula: EQU M.sub.2/n O, Al.sub.2 O.sub.3, xSiO.sub.2
where M represents a cation of valence n, such as for example an alkaline, an alkaline-earth or an organic cation, x may range, according to the structures, between two and infinity, in which case the zeolite is a microporous silica.
Although numerous zeolites of the aluminosilicate type do exist in nature, the search for products with new properties has led to the synthesis of a large variety of these aluminosilicates of zeolitic structure including mordenites. Mordenite is a zeolite which crystallizes in the orthorhombic system with crystalline parameters a, b, c respectively close to 18 1, 20.5 and 7.5 Angstroms (1 Angstrom=1 .ANG.10.sup.-10 meter) Its pore structure is monodimensional, the opening of the pores being delimited by cycles with 12 oxygens.
One of the first mordenite syntheses has been reported at the beginning of the sixties (A. KEOUGH and L. SAND, J. Am. Chem. Soc., Vol. 83, 1961, p. 3536): this synthesis is performed in an alkaline medium, the compensation cation being Na.sup.+. The ratio Si/Al of this synthetic mordenite is close to the value known for natural mordenite, that is to say 5. Contrarily to the mordenites of natural origin, this synthetic variety belongs to the type with wide pores, that is to say that it has adsorption properties in accordance with its crystallographic structure. In an alkaline medium and in the absence of any organic structurers, the maximum ratios Si/Al are close to 10 (0. WHITTEMORE, Am. Mineralogist, Vol. 57, 1972, p. 1146 P. BODART, J. B. NAGY, E. DEROUANE, Z. GABELICA, Stud. Surf. Sci. Catal., Vol. 18, 1984, p. 125). The use of cations or of organic molecules in the alkaline reaction mixture has allowed to synthetize mordenites with a ratio Si/Al higher than 10. It should be noted that, in these syntheses in the presence of organic agents, the sodium cation is always associated with the synthesis medium. Among these syntheses in the presence of organic compounds, the following patents can for example be cited : JP58 88,118, US-A 4,052,472, EP-A 80,615, US-A 4,525,466.
All the mordenite type zeolites which have been prepared until presently have been synthetized in a conventional medium, that is to say in an alkaline medium with a pH value generally higher than 9, a medium in which the mobilizing agent of the silica is the OH.sup.- anion. Another synthesis medium of the zeolites has been discovered recently : it is the fluoride medium, in which the mobilizing agent of silica is the F.sup.- anion; in this medium, the pH value is generally lower than 10 (see for example J. L. GUTH, H. KESSLER and R. WEY, Proc. 7th Int. Zeolite Conf., Tokyo, August 17-22 1986, p. 121). The synthesis of a limited number of zeolitic structures has already been successful in this new medium, as for example MFI (French Patent Application 88/09,631) and ferrierite (French Patent Application 86/16,362).
In relation to the alkaline synthesis medium (OH.sup.-), the fluoride medium shows a certain number of very appreciable advantages. In fact, in an alkaline medium, most synthetized zeolites are metastable : more stable solid phases are therefore likely to appear during the synthesis and unwanted phases may be precipitated. This difficulty increases when the amounts to be prepared are higher, that is to say by passing from the laboratory stage to the industrial stage. Besides, these metastable zeolites in the basic reaction medium are obtained only through a strong supersaturation of active species in the medium. This causes a rapid nucleation and consequently leads to crystals with small sizes, the average dimensions of these crystals ranging around one micrometer. Developing crystals with a larger size is therefore difficult in a basic medium. But, in certain applications, it may be interesting to have crystals with a larger size in order to preserve for example the thermal stability of the solid.
Besides, the chemistry of the aluminosilicate species in a fluoride medium is totally different from that which is known in the conventional alkaline medium. Syntheses in a fluoride medium may thus lead to solids the local Si-Al distribution of which in the structure is different from that which is known for zeolites synthetized in a conventional medium. This point is of particular importance in the case of mordenite. As a matter of fact, the aluminum is preferably located, for mordenites known in prior art, in the 4 membered rings of the structure (V. GRAMLICH, Ph.D, Dissertation ETH n.degree. 4633, Zurich, 1971; G. DEBRAS, J. B. NAGY, Z. GABELICA, P. BODART, P. A. JACOBS, Chem. Lett, 1983, p. 199); pairing 2 atoms of aluminum in each cycle is thermodynamically favored (E. G. DEROUANE, J. G. FRIPIAT, Proc. 6th Int. Zeolite Conf., Reno, USA, 1984, p. 717). This situation has two drawbacks : on one hand, the proximity of the atoms of aluminum tends to limit the force of the acid sites of the H forms and, on the other hand, the presence of such Al--Al pairs, even in the dealuminized H forms, is translated into poor selectivities in certain reactions for which the bi-site reactions should be banished (example of the isomerization of the aromatic C8). Preparing mordenites with a lower content of Al--Al pairs thus appears as a particularly interesting way of improvement.